The present invention relates to a method for operating power semiconductors.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
The electrical power loss in converters, which are suited for use for example in electric or hybrid vehicles for operating electric machines, is mainly generated or determined by their power semiconductors or power semiconductor modules. Aside from the types of power semiconductors used, for example IGBTs (Insulated Gate Bipolar Transistor) which are frequently used, the gate activation method has a decisive influence on the degree of the electrical power loss and thus on the efficiency of the converter. Closer evaluation of the switching behavior of power semiconductors indicates that a comparatively long duration in particular of the switch-off process of the power semiconductors represents a significant part of the entire electrical losses when switching power semiconductors.
During the switch-off process, the collector current of the power semiconductor, which flows through the power semiconductor in the conducting state, is reduced as a function of its rate of current rise, which results from the derivative dlc/dt of the collector current with regard to time t, until it assumes a value approaching zero. A small leakage current of the collector current can if necessary continue to flow. The rate of current rise of the collector current, in other words the gradient of the edge of the collector current, is also identified as a placeholder for the switch-off speed, which determines the duration of the switch-off process for the power semiconductor. It can be controlled by way of the gate activation of the power semiconductor. Depending on the switch-off speed, the collector-emitter voltage increases, as a function of an inductance L present on the direct voltage circuit (also known as inductance of the commutation circuit), beyond the direct voltage present on the intermediate DC circuit, wherein the collector-emitter switch-off overvoltage ΔVCE which results therefrom is produced in accordance with the following formula:ΔVCE=L*dlC/dt. 
For the power semiconductors, here in particular IGBTs, a maximum blocking collector-emitter voltage is specified in each case by the manufacturers, which if exceeded is expected to lead to the destruction of the power semiconductor. This maximum blocking collector-emitter voltage, often also referred to as nominal block voltage, is specified in the manufacturer's data sheet usually at a junction temperature of the power semiconductors of +25° C. Moreover, in respect of the junction temperature of the power semiconductors, reference is mainly made to the temperature of the power semiconductors or to the temperature on the power semiconductors. The power semiconductor must accordingly be used and operated such that a direct voltage VDC appearing on the intermediate DC circuit, which is still applied with the collector-emitter switch-off overvoltage ΔVCE during the switch-off process of the power semiconductor, does not exceed the maximum blocking collector-emitter voltage VCES, as described by the following formula:VCES>VDCΔVCE.
Accordingly, the rate of current rise of the collector current is in particular to be limited during operation of the power semiconductors regarding the respective value of the direct voltage on the intermediate DC circuit if necessary, in order to observe the indicated condition for the switching-off of the power semiconductor. This condition thus has a non-negligible influence on the duration of the switch-off processes of power semiconductors for many of its applications.
A robust design of the power semiconductor previously is currently accomplished in that the maximum collector-emitter voltage developing on the power semiconductor for a switch-off process at a maximum direct voltage on the intermediate DC circuit and at a maximum current (in other words also collector current through the power semiconductor), the maximum blocking collector-emitter voltage predetermined by the manufacturer have to be observed.
For IGBTs which are operated on an intermediate DC circuit with a higher voltage, the maximum blocking collector-emitter voltage VCES is specified for instance with 650V at an ambient temperature of +25° C.
When such power semiconductors are now intended to be used in converters in order to drive electric or hybrid vehicles for instance, the power semiconductor must in most instances also be configured for negative temperatures. In accordance with manufacturer specifications, the maximum blocking collector-emitter voltage is specified with just 605V at a temperature of −40° C. for instance. This collector-emitter voltage, which is now reduced for the blocking ability of the power semiconductor, which primarily applies at lower temperatures on the fringe of the usability of the power semiconductor, is often defined by the user to include the entire temperature range.
It would therefore be desirable and advantageous to provide an improved method which ensures a reliable operation of power semiconductors over a defined temperature range, wherein the electrical losses can be reduced during the switch-off processes of the power semiconductors